If the saints are God’s works of art, then Mary is God’s masterpiece. She is the highest of the saints, the greatest single creature ever made by God. Mary has an absolutely unique vocation that will never be repeated: She is the Mother of God. When the Blessed Trinity decreed that God the Son would be conceived, be born, live a human life, suffer, die, rise, and ascend into Heaven for the salvation of the human race, that same decree chose the Virgin Mary to be the Mother of God the Son. Since her Son, Jesus, is truly God and she is truly his Mother, she is the Mother of God.
The fact that Mary is the Mother of God is the basis for all her other special privileges (e.g., being preserved from all stain of sin and being assumed into Heaven prior to the Resurrection of the dead). Catholics owe a special kind of honor to Mary because of her unique role in our salvation. God did not force Mary to become pregnant with Jesus (think for a minute about what that would mean), rather Mary freely chose to cooperate with God’s plan (Lk 1:38). In doing this, she became the first and best disciple of Jesus: the first to know God made flesh and the first to cooperate with his mission on earth. Remember: Jesus is truly God and, through Mary, became truly man. So, Jesus himself honors Mary (otherwise he would be breaking the commandment to “honor your father and mother”). Jesus himself was obedient to Mary (Lk 2:51). That’s right: God himself was obedient to Mary, not because he had to be, but because in his great humility (Philippians 2:5-7), he freely chose to be.
Mary is the Ark of the New Covenant. Whereas the old Ark held the word of God on stone tablets, Mary carried in her body the Word of God made flesh. The old Ark held the manna from Heaven, but Mary carried the Body of Christ, the true bread come down from Heaven. Read the Old Testament, and you will see how much respect was paid to the Ark of the Covenant. How much more is owed to the Ark of the New Covenant!
Mary is the Queen of Heaven (Rev 12:1) because she is the mother of the king (Rev 12:5). If you want to see how important the mother of the king is in the Bible, read 1 Kings 1:15-16 (where Bathsheba, the wife of David the king, asks him for a favor) and then compare it to 1 Kings 2:18-20 (after David has died and Bathsheba, the mother of King Solomon, asks him for a favor). In the kingdom of David, the mother of the king held a very influential position indeed (this is why throughout 1 Kings, the mothers of David’s successors are named). Jesus is the ultimate successor of David, the final king in the line of David (Lk 1:32). So, Mary is the ultimate and final queen in his kingdom.
Catholics do not worship Mary (which would be to give to her the honor due only to God), we simply fulfill the biblical command (Lk 1:48) to acknowledge the great things that God has decided to do through her. At first glance, it may seem that the Bible says little about Mary, but that only shows how unfamiliar we really are with the Scriptures. If we look at the role that Mary has and how this role is described throughout the Scriptures, we will find that the Bible speaks powerfully about our Blessed Mother. A good book to get started in learning about this is Hail, Holy Queen by Scott Hahn.